(1) Field of the Invention
The present invention relates to a mobile communication system having a PTT (Push to Talk) function.
(2) Description of the Related Art
In recent years, some mobile stations in a CDMA system have a PTT function, which is for transmitting voice to other mobile stations by just pushing a PTT (Push to Talk) button like a transceiver. During a PTT mode, an IP address is allocated to a mobile station having the PTT function, and when the PTT button of the mobile station is pushed, the user's voice is taken in, packetized, and transmitted to another mobile station having an IP address, via an IP network (e.g. Japanese published unexamined application No. 2000-209147).
However, in the PTT mode, it takes a long time for voice packets transmitted from one mobile station to reach another mobile station.
A major cause of this problem is that procedures for transmitting voice packets include a procedure for transmitting messages from a base station to a mobile station on a Paging Channel, such as an allocation of a Traffic Channel for transferring voice packets and notification of incoming voice packets. This procedure for transmitting massages is performed only at a predetermined frequency, not at all times.
More specifically, the CDMA system provides a Slotted mode for saving power for a mobile station. During this mode, the Paging Channel, which is for notification of control information, pages, and so on, is divided into slots (called “Paging Channel Slots”), each of which is 80 ms long. A mobile station monitors only a specific Paging Channel Slot in cycles (called “Slot Cycles”). While a mobile station operates in this mode, a base station accordingly uses only the specific Paging Channel Slot in cycles for transmitting messages, such as control information and notification of incoming calls. The duration time of the Slot Cycle in idle mode is generally 5.12 sec. Therefore, it takes a half of the duration time on average, that is to say, it takes 2.56 sec to transmit one message. The length of the duration time is not a major problem in the case of transmitting messages for notification of incoming calls. However, it is a big problem in the PTT communications, because every time a user pushes the PTT button to speak, the user has to wait 2.56 sec on average for each Paging Channel Slot in both transmission and reception. The duration time hinders a seamless conversation between users.